1. Field of the Invention
This invention relates, in general, to a monitoring apparatus for a fluid flow system, such as a high pressure liquid chromatography system, and to methods for its use.
2. Description of Related Art
High pressure liquid chromatography (HPLC) generally requires the components of a sample to be separated or analyzed be dissolved in a mobile phase liquid, termed an eluent, and conveyed by that liquid to a stationary phase, that is, a chromatography column. HPLC eluent delivery systems are used to supply the liquid and deliver the liquid, with dissolved sample, to the column.
Specially designed HPLC pumps are used to deliver the liquid at precisely controlled flow rates in a smooth and uniform manner. In this manner, the composition of the eluent can be varied in controlled amounts whereby the strength of the mobile phase can be increased linearly or in increments in elution of the sample. This process is commonly referred to as gradient elution. During an HPLC gradient elution, the composition of the eluent is changed temporally to effect the elution of components in the sample with a wide range of affinities for the stationary phase.
As the composition or gradient profile of the eluent changes, the viscosity of the eluent also changes. For example, an eluent composed of water and methanol (MeOH) has the interesting property that the viscosity is actually at a maximum close to the point where the solvents, water and methanol, are nearly equally proportioned. Although water is more viscous than methanol, the maximum viscosity of an H2O/MeOH gradient is not reached at 100% water, but at a mixing ratio of 60:40 of water to methanol. This viscosity maximum is about 60% higher than the viscosity of water alone.
Water and acetonitrile (ACN) are also commonly used together as an eluent. The viscosity of water and acetonitrile differs by as much as 250%. However, the viscosity ratio of water and acetonitrile is similar to that of water and methanol. Thus running an H2O/ACN gradient generates a S-shaped pressure profile rather then a constantly decreasing curve. The S-shaped H2O/ACN pressure profile is shown in FIG. 1.
Micro-flow processors for varying the composition of the eluent are known. An exemplar of a prior art splitter is disclosed by European Patent No. EP 0 495 255 B1 to LC Packings, which patent shows a micro-flow processor having two restrictor portions which are formed by capillaries of different internal diameters and lengths. The flow splitter of the European ""255 patent includes two capillaries with a solvent resistance ratio defined by the split ratio, e.g. 1:70.
Often, the method of splitting the flow by two capillaries whose length is proportional to the split ratio does not work in practice, because the back pressure within each capillary depends on the viscosity of the eluent. If the eluent composition differs in the two capillaries, the viscosity of the eluent differs and, thus, the splitting ratio varies as well.
Disadvantageously, the residence time of the gradient profile in the capillary coupled to the column must be similar to that of the gradient profile in the capillary coupled to a waste line in order for a flow splitter to function correctly when temporal eluent gradients are employed. Otherwise, hydraulic resistance of each capillary leg or restrictor portion will change at dissimilar rates and the flow rates through both capillary legs will change during the gradient development.
What is needed is a method and apparatus for monitoring the actual micro-flow through a flow splitter which overcomes the above and other disadvantages of known flow splitters.
In summary, one aspect of the present invention is directed to an apparatus for monitoring flow within a high pressure liquid chromatography system, the apparatus including a pump for supplying a system flow of eluent to the system, a chromatography column, a flow splitting device including a first restrictor and a second restrictor, the first restrictor fluidly connecting the pump and the chromatography column and the second restrictor fluidly connecting the pump to a waste line, a first fluid pressure sensor monitoring a system fluid pressure upstream of the first restrictor, and a second fluid pressure sensor monitoring a column fluid pressure downstream of the first restrictor.
Preferably, the apparatus includes a supply line fluidly connecting the pump to the splitting device and a feed line fluidly connecting the first restrictor to the column. The first fluid pressure sensor coupled to the supply line for monitoring the system fluid pressure and the second fluid pressure sensor is coupled to the feed line for monitoring the column fluid pressure.
Preferably, in one embodiment, the apparatus further includes a calculating device for calculating a quotient Q based upon the system fluid pressure and the column fluid pressure as follows:
Q=Psys/(Psysxe2x88x92Pcol)
wherein Psys is the system fluid pressure and Pcol is the column fluid pressure.
Preferably, the apparatus includes a regulating device for regulating a required system flow necessary to provide a desired column flow based upon the system fluid pressure and the column fluid pressure.
In one embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97(1+S)xc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In another embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97(1+Sxc3x97Q)
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In yet another embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97Sxc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
Another aspect of the present invention is directed to an apparatus for monitoring flow within a high pressure liquid chromatography system, the apparatus including a pump for supplying a system flow of eluent to the system, a chromatography column, a flow splitting device including a first restrictor and a second restrictor, the first restrictor fluidly connecting the pump and the chromatography column and the second restrictor fluidly connecting the pump to a waste line, a first fluid pressure sensor monitoring a system fluid pressure upstream of the first restrictor, and a differential fluid pressure sensor monitoring a pressure differential between the system fluid pressure and a column fluid pressure downstream of the first restrictor.
Preferably, in another embodiment, the apparatus further includes a calculating device for calculating a quotient Q based upon the system fluid pressure and the pressure differential as follows:
Q=Psys/Pdiff
wherein Psys is the system fluid pressure and Pdiff is the pressure differential.
In one embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97(1+S)xc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In one embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97(1+Sxc3x97Q)
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In one embodiment, the apparatus includes a regulating device for regulating a required system flow Fsys necessary to provide a desired column flow Fcol as follows:
Fsys=Fcolxc3x97Sxc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
Yet another aspect of the present invention is directed to a method for regulating a desired column flow within a high pressure liquid chromatography system including the steps of supplying an eluent via a supply line to a flow splitting device at a system flow rate, distributing the eluent from the splitting device via a feed line to a chromatography column, monitoring a system fluid pressure in the supply line, monitoring a column fluid pressure in the feed line, and calculating a quotient Q based upon the system fluid pressure and the column fluid pressure.
In one embodiment, the calculating step is accomplished by calculating the quotient Q as follows:
Q=Psys/(Psysxe2x88x92Pcol)
wherein Psys is the system fluid pressure and Pcol is the column fluid pressure.
In another embodiment, the calculating step is accomplished by calculating the quotient Q as follows:
Q=Psys/Pdiff
wherein Psys is the system fluid pressure and Pdiff is the differential pressure between the system fluid pressure and the column fluid pressure.
Preferably, the method includes the step of regulating a required system flow based upon quotient Q.
In one embodiment, the regulating step is accomplished by regulating the required system flow Fsys as follows:
Fsys=Fcolxc3x97(1+S)xc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In another embodiment, the regulating step is accomplished by regulating the required system flow Fsys as follows:
Fsys=Fcolxc3x97(1+Sxc3x97Q)
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
In yet another embodiment, the regulating step is accomplished by regulating the required system flow Fsys as follows:
Fsys=Fcolxc3x97Sxc3x97Q
wherein Fcol is the desired column flow, S is the split ratio of the flow splitting device, and Q is the quotient.
Preferably, the method includes the step of adjusting the system flow rate to provide the desired column flow rate.
An object of the present invention is to provide a method and apparatus for monitoring the actual micro-flow through a splitter device of a fluid flow system.
Another object of the present invention is to provide a method and apparatus for monitoring actual micro-flow in order to minimize flow rate fluctuations in the fluid flow system.
Yet another object of the present invention is to provide a method and apparatus for monitoring the micro-flow of a mobile phase through a splitter device and to a stationary phase of an HPLC system with acceptable accuracy at any interval of time.
The method and apparatus for fluid flow control of the present invention has other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain the principles of the present invention.